Description (adapted from the application): The Administrative Core of this Chronic Fatigue Syndrome (CFS) Cooperative Research Center will consist of the project leaders of the 3 Projects. This group also constitutes the Executive Committee. An Oversight Committee, comprised of Center staff members involved in subject recruitment and performance of the individual research projects, meets on a bi-weekly basis. The functions of the Administrative Core are: 1) administration/coordination of ongoing Center activities; 2) integration of this Chronic Fatigue Syndrome Cooperative Research Center, with communication of information among the collaborating investigators and institutions; and 3) internal program review planned to permit interaction with the NIAID Scientific Coordinator; and 4) administration and approval of developmental funds for pilot studies. The Patient Recruitment Core will be responsible for recruiting new CFS patients and healthy controls to participate in each of the three projects; performing physical and psychological evaluations on all new participants; and is responsible for administering neuropsychological testing. In addition to ensuring that patients meet the inclusion criteria for research, the classification of cognitive impairment will be determined. The Data Analytic Core will provide biostatistical and data management services to each of the research projects.